The invention relates to making composite material parts and particularly to making fiber reinforcing textures for such parts.
The field of application of the invention is more particularly making thermostructural composite material parts, i.e. parts made of a material having mechanical properties that make it suitable for constituting structural elements and having the ability to conserve these properties at high temperatures. Thermostructural composite materials are typically carbon/carbon (C/C) composite materials having carbon fiber reinforcement densified by a carbon matrix, and ceramic matrix composite (CMC) materials having refractory fiber reinforcement (carbon fiber or ceramic fiber) densified by a ceramic matrix. Thermostructural composite material parts are used in particular in the fields of aviation and space.
For parts made of composite material that present a certain thickness, it is common practice to make the reinforcing texture out of a plurality of superposed layers that are bonded together so as to avoid the layers separating, in particular by performing three-dimensional weaving.
Furthermore, in particular for a composite material part that is obtained by densifying the fiber reinforcing texture by chemical vapor infiltration (CVI), it can be useful to provide easy access for the gas to the core of the fiber texture or to ensure that the fiber texture presents porosity that is relatively uniform in order to achieve densification that departs as little as possible from being uniform. In the event of access being difficult to the pores in the fiber texture, or in the presence of pores of very different sizes, then the smaller pores become filled in more quickly and a strong densification gradient is inevitable, thereby affecting the properties of the composite material.
Document EP 0 489 637 proposes making a fiber reinforcing texture for a thermostructural composite material part by using a yarn made up from discontinuous fibers with substantially no twist, yarn cohesion being provided by a covering yarn. The texture can be made by three-dimensional weaving. The covering yarn is made of a temporary or sacrificial material that is eliminated after weaving, thereby allowing the discontinuous filaments to bulk up, which enhances subdivision of the pores of the woven texture and, in combination with three-dimensional weaving, encourages access for the gas to the core of the fiber structure during subsequent densification by CVI.
Nevertheless, when fabricating composite material parts that are to present a surface state that is very smooth, it is necessary to trim the surface after an initial stage of partial densification, also known as a consolidation stage, in order to eliminate the irregularities created at the surface by the bulking up of the discontinuous filaments that are released once the covering yarn is eliminated. It can also be necessary to add a ply of two-dimensional cloth prior to continuing densification in order to obtain a desired surface state.